The objective of this study was to improve the properties of polypropylene-zeolite composites by enhancement of the interphase between polypropylene and zeolite. Surface treatment of zeolite was applied for modification of interfacial interactions between zeolite and polypropylene. Surface treatment of natural zeolite was carried out with (3 wt%) polyethyleneglycol (PEG) and three different silane coupling agents namely, 3-aminopropyltriethoxysilane (AMPTES), methyltriethoxysilane (MTES), and 3-mercaptopropyltrimethoxysilane (MPTMS) at four different concentrations (0.5, 1, 1.5 and 2 wt%) to find suitable surface modifier for improving filler compatibility and mechanical properties. PP composites containing (2, 4 and 6 wt%) untreated or treated zeolite and plasticizers: dioctylphthalate (DOP) or epoxidized soybean oil (EPS) were prepared by extrusion technique. The effects of the modifications and zeolite content on the thermal, mechanical, structural and physical properties of PP composite were investigated. The contact angle measurements and FTIR analyses of untreated and silane treated zeolite samples and water sorption results of PP-zeolite composites showed that hydrophobicity of zeolite significantly increases with surface modification. Thermal analyses indicated that the addition of zeolite and silane treatment to the PP-EPS matrix did not change the melting and degradation temperature of the composites. However, these modifications were found to increase the crystallinity and crystallization temperature of the composites due to the nucleating effect of the zeolite. The mechanical properties of PP composites containing silane treated zeolite indicated significant improvements compared to the composites containing untreated filler. The most enhanced dry and wet mechanical properties were observed for PP composites containing 1 wt% AMPTES treated zeolite. The effect of interfacial interactions and adhesion between zeolite and PP was evaluated by various semiemprical equations: Pukanszky model, Nicholais and Narkis model and Nielsen model. The improvement in adhesion between silane treated zeolite and PP was also confirmed from these models. Moreover, the water sorption and mechanical test results, as well as scanning electron micrographs and optical micrographs of the composites verify that silane coupling agents enhanced compatibility and interfacial adhesion between zeolite particles and PP matrix strongly led to an improvement of the mechanical properties of the composites. Consequently, 1 wt% AMPTES was proposed to be the most appropriate surface modifier by considering the water sorption results, thermal, mechanical and microstructure analyses of PP-zeolite composites.